While some telephone exchanges or central offices still operate with electromechanical call-signal generators, others use more recently developed electronic equipment for this purpuse. The two types of call signal respectively emitted by electromechanical and electronic generators differ from each other in both amplitude and frequency. Thus, the electromechanically generated call signal lies in a low audio-frequency range, usually between 20 and 50 Hz, and is sent out with a high voltage level which may be on the order of 100 volts; the electronically generated call signal, on the other hand, lies in a higher audio-frequency range (e.g. between 400 and 500 Hz) and is emitted with an amplitude on the order of only a few volts.
Since a given subscriber station may have to be switched from one central office to another, or since an existing central office serving a large number of subscribers may change over in the course of modernization from electromechanical to electronic call-signal generation, it is desirable to equip subscriber stations with ringing-signal generators adapted to respond to either type of call signal.
Ringing circuits are known (e.g. from Italian patent application No. 19557-A/77) which check the frequency and the duration of incoming signals in order to determine the presence or absence of a call signal. We are not aware, however, of any such circuitry able to differentiate on the basis of amplitude and frequency between electromechanically and electronically generated call signals.